Prepare for Article 50's ugly big brother
Most people have never heard of Article 127 – the trigger to leaving the European Economic Area – but it is Article 50's bigger and uglier brother. And potential litigation about Article 127 will make the Article 50 case look like a walk in the park.
Now that the Supreme Court has decided MPs need to vote before Article 50 can be triggered, attention has turned to the Brexit bill, which has been rushed through parliament to make sure the government is able to meet the March deadline for service of the UK's "Dear John" letter to the EU.
Naturally, there are attempts to thwart the legislation. Between them, the Scottish Nationalist Party, the Greens and rebel Labour MPs tabled more amendments than there were words in the bill.
Having gone through the Commons we will see what damage the Lords tries to inflict. If peers throw it out then that would represent the biggest constitutional crisis since 1909, when they refused to approve a budget.
The proposal made then by David Lloyd George, the prime minister, was that if they did not make the law he would create 400 new peers to do his bidding. The law went through.
But if you think you are witnessing a battle now, think again. Once Article 50 is triggered the fighting will really start. It will be about the type of Brexit we have: hard or soft. Are we going to stay in the single market or do we come out or will we be somewhere in between?
Most Brexiters and the government want a hard Brexit. If they get their way and we come out of the single market, we will need to consider how we do that.
The legal structure of the single market is contained in a treaty called the European Economic Area Agreement. Membership is open to all EU members and also to Norway, Liechtenstein and Iceland. The UK joined the EEA by signing the agreement and by enacting the European Economic Area Act.
The process by which a country leaves the EEA is set out in Article 127, similar to Article 50. Article 127 provides "each contracting party may withdraw from this agreement provided it gives at least twelve months' notice in writing to the other contracting parties".
The government could argue that it can trigger Article 127 under the royal prerogative. The strength of feeling in parliament and in the country about whether the UK should stay in the single market seems deeper than the feeling about membership of the EU.
When we reach this point there will be litigation over Article 127, and if the court decides as it did for Article 50 that parliament must vote then that will be the biggest political fight since the English Civil War.
This article was first published in The Times Brief on 2 February 2017.